Measuring device and method of measuring



Aug. 14, 1951 A. M. MaGMAHoN ET AL 2,564,626

MEASURING DEVICE AND METHOD OFIMEASURING Filed March l0, 1945 Patented Aug. 14,".1951` nervio Ann ivrit'rn'on or MnAsu MEASURING Andrew MacMahon,

Arthur H. Snell, Gak Ridge,` Tenn., assignors to the United VStates of by the' UnitedStates Atomic Energy' Coinjiission ING

Houston,- Tex.,` and` America as represented Appiiation March 10, 1945,i serial No.r- 582,092

Our invention relates to a method and apparatus for measuring integrated neutron activity, and is particularly useful in measuring the in@- tegrated neutron activity in a neutronic reactor, although it may also be used for measuringintegrated neutron activity resulting from other neutron sources.

In the past, various methods and devices have been used for measuring integrated neutron activity, that is, the product of time and neutron density, such as, ionization chambers coupled with electrometer and integrating circuits for measuring ionization currents caused by a neutron stream, but such devices have the disadvantage of being diiic'ul't to calibrate and to keep in adjustment dueV to cathode drift and other similar' disturbing transient elects. Furthermore, such electronicapparatus is delicate, sensitive in operation, and relatively expensive.

An object of our invention'is to provide' a method and apparatus for Ineasuring'integrated neutron activity that is devoid of theV above mentioned disadvantages and' that is relatively simple, rugged, relatively inexpensive and reliable' in operation.

A more specic' object of our invention is to provide a method and apparatus for measuring' the integrated neutron activity resulting from a neutron source, such as, for example, a neutronic reactor.

In general our invention involves the principle of inserting into a neutron stream having' the intensity under measurement, an element that' will undergo-nuclear reactions and transmutation by neutron absorption and radioactive'k decayA to produce an end product comprising a dinerent element having anelectrical conductivity markedly different from the conductivity of the original element, and to utilize this change in conductivity as a measure ofthe integrated neutron activity or intensi-ty to which the original element was exposed.

Other objects and advantages will become more apparent from the following description taken together with the drawing,- in which the single ligure isa schematic diag-'ram of a Wheatstone loridge circuit illustrating the teachings of vcit-1r invention and adopted for measuring integrated neutron activity.

Referring more particularly to the drawing, numerals I, 2, 3, and 4 denote resistance elements in the four legs of a Wheatstone bridge circuit. Resistance elementsf 2, 3, and i are .non-e inductively wound standarcll resistances, the re sistance element 2 being variablev and` being usedA as a temperature compensating resistor. Ite-V ,710 claims. (C1. 25o-sas) `sistance element or speciment is the sensitive unit whose resistance value, or change in resistance resuiung fromY neutron tdmbardnint, is to be measured, as will appear' hereinafter;

.y Numeral denotes 'a g'alv'anmeter for indicatingA unbalance in the i'v'heatstone bridge, or, if de"- sired, may comprise a measuring ir'istrumentl or" recorder for measuring the amountbf the' un-v balance. A low potential source, such as a bat-2 tery I, is used to energize they bridge circuit@ In order tol measure integrated neutron activity, that is', the product of neutron densityV and time; resistance element I Y is placed in the path of a neutron stream or beam as illustrated by the arrows in the drawing. y In actual practice this mayl be accomplished, for example, by placing resistance element I inside or adjacent tothe periphery of a neutroni'c' reactor in a 'neutron' beani the intensity of which is under measuresment and the integrated neutron activity of' which is to be measured over a period of time;

Broadly speaking, a neutronic reactor isa devic'e comprising a plurality of uranium or other 'ssion'able' elements dispersed in a neutron slow`VY ing material or moderator, such asg-raphite orheavy water, capable of slowing fast neutrons to thermal energy, and that is built of suiii'cient.

size toA produce a self-sustaining nuclear chain'- reac'tion. Our invention is particularly appli-V cable for use ingneutronic reactors capable of?.I having high neutron density and of being oper; ated over continuous periods' of time.l

To measure the integrated neutron activity'- in such a neutroni@ reactor, the bridge circuit is first balanced and thereafter resistance element I is placed inside the reactor, that is, in the neutron stream. By noting the temperaturey of resistanc'ze element I as determined by a thermocou'ple' or?" other suitable temperature' indicating instrument at the time of reading of the new value of resist-1 ance element I, as will appear more clearly here@ inafter, a' correction may be applied to variable v resistance element 2 to compensate for the change' of resistance of element I du'e to increasein temperature of element I because of itsv close` prez'iirni-ty4 to the heated materials forming the-'- neutronic reactor. The remainder of the Wheat# stone'v bridge circuit is located exteriorly ofthe' reactor and has suitable conductors extending through the outer wall of theA neutronic reactor"4 that are connected to the resistance elements 2fand 4' by' Phosphor bronze. lead-'in wires E and 9; for example, contained' in tube ti.

- The outstanding feature of our invention em;i bodies the specimen or resistance element I'f and. its application. Resistance element I comprises.

o-FFICE any suitable material or element that undergoes an appreciable change in resistance when transmuted to a different material or element by neutron capture as the result of exposure to neutrons overA aperiod of time. Resistance element I, for example, may be made in the form of a helical-spiral coating inside a glass vacuum tube S. if the resistance material is one which cannot be readily formed as a metallic sheet or strip, such as, forexample, boron. A vacuum of about l3mm.I-Ig is suitable. It is known that boron when subjected to continued neutron bombardment will undergo a nuclear change and transmute to lithium which has an appreciably different electrical resistance than boron, vwhich change in resistance may be readily and accurately measured by a Wheatstone bridge or other resistance measuring device. A typical reaction may be represented as follows:

An alternate arrangement is to commence with a helical-spiral strip of lithium as a coating in vacuum tube 6 and to subject the lithium to a stream of neutrons so as to convert it directly to helium and hydrogen. A typical reaction may be represented as follows: 3Li6+0n1- 1H3+2He4. In this case, it should be noted that prior to complete conversion, only a portion of the lithium is' transmuted to hydrogen and helium consequently there will remain after bombardment a lithium helical-spiral or smaller cross-sectional area, and therefore, of higher resistance than the original spiral. Therefore, by measuring the amount of increase in resistance ofthe remaining lithium helical-spiral after a predetermined bombardment with neutrons, it is possible to obtain an indication of the integrated neutron activity to which the strip was exposed over any given time period.

A further modification may comprise a strip of aluminum used as resistance element l in which case vacuum tube 6 is not needed. When the aluminum strip is bombarded by neutrons, it converts or transmutes to silicon having an appreciably lower electrical conductivity than the aluminum. Likewise a strip of lead may be used instead of aluminum or lithium. The lead will convert or transmute to bismuth again changing the electrical resistance. In general, any other material or element whose resistance changes appreciably, that is by readily measurable amounts, when bombarded with neutrons, may be used.

More specifically, the criteria involved in the selection of a suitable material forming the sensitive unit, that is resistance element l, for use in. measuring the integrated neutron activity in a neutronic reactor are as follows: (l) the capture cross-section for thermal neutrons, that is to say, the absorption ability for thermal neutrons should be as great as is consistent with the normal operation of the reactor; (2) the related nuclear reaction should be as simple as possible giving rise to a product which dominates all others in effecting change of electrical resistance; (3) the product should have a very long life in comparison with the period of operation of the reactor, (4) the difference between the electrical resistivities of the product and parent substance should be as large as possible; and (5) the physical properties of both the prod uct and the fparent substance should be such as to insure the stability of the electrical resistance of the sensitive unit, namely resistance element l, during periods of measurrnents.

It will be seen, therefore, that we have pri vided an efficient apparatus and method of measuring integrated neutron activity, including an element that will undergo nuclear reactions to produce an end product having an electrical conductivity markedly different from the conductivity of the element before being subjected to neutron bombardment, and we have utilized this change of conductivity or resistance as a measure of the integrated neutron intensity.

It should be noted that the circuit shown and the resistance materials described are merely illustrative and not limiting with respect to the present invention. Alternate materials having the above mentioned characteristics will readily suggest themselves to others skilled in the art after having had the benefit of the teachings of our invention. Likewise resistance measuring devices other than a Wheatstone bridge may be used for measuring the above described change in electrical resistance due to transmutation accompanying neutron bombardment. In fact, it may be possible even to analyze the transmuted elements or end product by visual means only, such as a microscope, or even by observation by the naked eye for changes in color, or other physical properties. By testing for changes in physical or chemical properties, the end product may be identified. For this reason, the invention should be restricted only insofar as set forth in the following claims.

We claim:

1. The method of measuring integrated neutron activity, comprising measuring the resistance of a solid elongated body, said body consisting of elements transmuting to gaseous elements under neutron bombardment, exposing the body to neutron radiation to obtain partial transmutation to a gaseous element, and measuring the electrical resistance of the body after such exposure.

2. The method of measuring integrated neutron activity comprising measuring the resistance of an elongated lithium body, exposing the lithium body to neutron radiation to obtain partial transmutation to gaseous elements, and measuring the electrical resistance of said lithium body after such exposure.

3. Apparatus for measuring integrated neutron activity comprising an electrical bridge circuit including, in one of its legs, a strip of lithium in the form of a coating that readily transmutes into gases of appreciably different electrical resistance than that of lithium when subjected to continued neutron bombardment.

4. Apparatus for measuring integrated neutron activity comprising an electrical bridge circuit including, in one of its legs, a strip of lithium in the form of a coating that readily transmutes into gases of appreciably different electrical resistance than that of lithium when subjected to continued neutron bombardment and including, in another leg, a temperature compensating resistor.

5. Apparatus for measuring integrated neutron activity comprising an electrical bridge circuit including, in one of its legs, an evacuated tube having an internal coating in the form of a strip of material that transmutes to a gas after being subjected to continued neutron bombardment.

6. Apparatus for measuring integrated neutron activity comprising an electrical bridge circuit including, in one of its legs, an evacuated tube having an internal coating in the form of a strip of lithium that transmutes to gas after being subjected to continued neutron bombardment.

7. The method of measuring integrated neutron activity comprising, measuring the conductivity of a thin strip consisting of elements transmuting into gaseous elements under neutron bombardment, exposing the strip to neutron radiation, and again measuring the conductivity of the strip.

8. The method of measuring integrated neutron activity comprising, measuring the conductivity of a thin lithium strip, exposing the strip to neutron radiation to obtain partial transmutation to gaseous elements and measuring the electrical conductivity of said lithium strip after such exposure.

9. The method of measuring integrated neutron intensity comprising the steps of applying an electrical potential to the ends of a thin electrically conducting strip consisting of elements which transmute into gaseous elements under neutron bombardment, and exposing the strip to neutron radiation to effect partial transmutation of the elements into gaseous elements, whereby the electrical conductivity of the strip calculated before and after irradiating the strip is a measure of the integrated neutron intensity.

10. Apparatus for measuring integrated neutron intensity comprising an evacuated tube, a

REFERENCES CITED The following references are of record in the ile of this patent:

UNITED STATES PATENTS Number Name Date 1,023,485 Thowless Apr. 16, 1912 1,229,740 Furstenau June 12, 1917 2,186,757 Kallmann Jan. 9, 1940 2,188,115 Kallmann Jan. 23, 1940 2,220,509 Brons Nov. 5, 1940 2,288,718 Kallmann July 7, 1942 OTHER REFERENCES Amoldi et al.: Proceedings of the Royal Society of London (A), vol. 149, 1935, pp. 522, 539, and 540.

Booth et al.: Proceedings of the Royal Society of London (A), vol. 161, 1937, pp. 248 and 254-260. 

